1. Field of the Invention
The present invention relates to relatively rigid structures made from cloth impregnated with a thermosetting compound, and more particularly to radomes of the fluted core type made from resin impregnated glass cloth.
2. History of the Prior Art
Radomes and other structural members made from woven materials such as glass cloth which are impregnated with a thermosetting compound are widely known and used. Examples of such radomes are provided by U.S. Pat. No. 3,871,001 of Myers (commonly assigned with the present application), U.S. Pat. No. 3,795,559 of Horn et al and U.S. Pat. No. 2,755,216 of Lemons. Composite aircraft radomes of this type have been manufactured utilizing three basic types of construction which include solid laminate, conventional sandwich and fluted core sandwich.
Solid laminate radomes are typically made from glass or polyester cloth. Because plies of the material must be added to enhance the structural characteristics of the radome, the result is often excessive weight as well as poor transmission characteristics. Some improvements have been realized in solid laminate radomes through the utilization of filament winding to orient the fibers, reduce the required material and substantially reduce the hand labor costs. Nevertheless, the inherent disadvantages in radomes of this construction type remain.
Typical radomes of conventional sandwich construction have glass skins impregnated with epoxy or polyester resin and bonded to cores of glass honeycomb, polyurethane foam or syntactic foam. The advantages of such construction include low cost, high strength, low weight and good transmission characteristics. A principal disadvantage of such construction lies in the tendency of such radomes to absorb and retain water. This becomes a problem as commercial aircraft routinely pass through the dew point several times during a flight day. The moisture which collects within the radome repeatedly freezes and thaws when subjected to such conditions. This coupled with some icing on the outer skin results in reduced electrical transmission properties and blind spots. Eventually, the moisture may cause failure at the core-skin interface due to the repeated wedging action which occurs with freezing and thawing.
Radomes of fluted core sandwich construction have been constructed using a variety of techniques. One such technique involves the use of an integrally woven three-dimensional fabric designed and woven for a specific radome application. This technique has the advantages of high strength, provision for moisture removal and minimization of icing through use of warm air ducting. Disadvantages include high cost, greater manufacturing difficulty, reduced radar transmission efficiency, expensive repairs and lack of means for a definitive vertical web quality verification.
As noted the fluted and honeycomb core radome constructions have a number of highly desirable features along with the disadvantages thereof. It would therefore be desirable to have available a new method of radome construction enjoying the advantages of the desirable features of both such methods while at the same time reducing cost, weight and some of the other disadvantages attendant in such constructions. Much of the high cost is attributable to high materials cost as well as extensive hand labor. Unitized web construction makes rejects a significant factor. Weight is related to the fabrication method and the design to achieve the necessary transmission characteristics. It is frequently necessary to add a second layer of fluted structure at predetermined angles to cancel a focusing effect caused by a web restricted flute orientation. Additional weight problems are often caused by the fiber weight used to achieve the integrally woven three-dimensional fabric, the variable resin content which results from a wet lay-up impregnation and the additional skins required to tie adjoining strips of three-dimensional fabric together.